1. Field of the Invention
The present invention relates to small transmissions such as used in riding lawn mowers and the like. More specifically, the field of the invention is that of shift shafts for such in-line transmissions.
2. Prior Art
In-line shift transmissions include a plurality of linked transmission shafts, with one of the shafts being a shift shaft supporting a plurality of rotatable spur gears. The transmission housing includes bearings which support the shift shaft at its opposite ends. The shift shaft includes axial channels in which keys are disposed for engaging internal keyways of the rotatable spur gears and thereby rotatably connecting the engaged spur gear to the output shaft. The key is made of a resilient metal including an elongate shaft disposed in the axial channel of the shift shaft and a head portion which is biased to extend into one of the keyways.
An in-line transmission is disclosed in U.S. Pat. No. 4,996,574, assigned to assignee of the present invention, the disclosure of which is explicitly incorporated by reference. The shifting mechanism of the '574 transmission includes a pair of axially movable shift keys disposed in the longitudinal channels of the shift shaft. The shift keys rotatably couple the shift shaft with one of the gears. Each key has a base portion engaging a shift collar which is axially slidable on the shift shaft. The collar has a peripheral groove receiving a pin of a shift fork, which can pivot within the housing and is connected to the shifting mechanism of the vehicle. The shift collar may be axially moved along a portion of the shift shaft by the pivoting action of the shift fork. The spur gears are disposed on a second portion of the shift shaft, and the resilient keys extend from the shift collar on the first portion of the shift shaft to engage a keyway of a spur gear on the second portion of the shift shaft.
The shift fork, pin, and annular groove permit the collar, and hence the keys, to be moved axially while the keys are at the same time rotating with the shift shaft. Each of the spur gears journalled on the shift shaft has on its inner circumference four circumferentially spaced keyways which may be engaged by the keys to rotatably couple the shift shaft with the selected spur gear. Consequently, as the keys are moved axially by the shift collar, the key heads successively engage and disengage keyways of adjacent spur gears, thereby coupling each spur gear to the shift shaft sequentially one at a time.
In order to provide the several shifting positions in the transmission disclosed in the aforementioned U.S. Pat. No. 4,996,574, the shift shaft extends across the length of the transmission housing. Further, in order to support the loading of the gears, the shift shaft must be made of material, for example steel, capable of withstanding the high loading forces from the transmission gears. However, the strong material which comprises the shift shaft is relatively expensive, thus increasing the cost of an in-line transmission.
In one known transmission, one section of the steel shift shaft has a reduced diameter and is fitted with a plastic sleeve which defines the axial channel around the reduced diameter portion of the steel shift shaft. This arrangement reduces the amount of steel required to make the shift shaft while still providing a seating for the collar and an axial channel for positioning the keys. However, the shift shaft still must extend across the length of the transmission housing which requires almost as much steel material as single piece shift shafts. Therefore, further reduction in the amount of steel material in the shift shaft is desirable.
What is needed is a less costly shift shaft for a transmission.
Also needed is such a shift shaft which reduces the amount of steel material without impairing the functionality of the shift shaft.